This invention relates generally to pressure sensors and more particularly to pressure responsive variable parallel plate capacitive transducers. Such transducers are shown and described, for example, in U.S. Pat. No. 4,716,492, assigned to the assignee of the present invention. A capacitive transducer is shown in the patent having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation on a ceramic base, with metal coatings deposited on respective opposing surfaces of the diaphragm and the base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electrical circuit connected to the transducer terminals is mounted on the transducer. A cup-shaped connector body of electrical insulating material is fitted over the electrical circuit and is secured to the transducer by a housing sleeve which has a port for exposing the transducer diaphragm to an applied fluid pressure. The diaphragm is movable in response to variations in pressure applied to the diaphragm to vary the capacitance of the capacitor and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
In order to maximize the economies of mass production to lower the transducer cost and thereby make such transducers economically feasible for a wide number of applications, including many previously served by low cost mechanical transducers, a standard size package is selected small enough to be received in a large number of applications yet large enough to provide a reliable signal. The size of the package determines the maximum size of the capacitor plates which, along with the gap between the plates, determines the capacitance signal. This results in limiting the size of the capacitor plates to a smaller size than would be ideal for many applications and relying on the electrical circuit to properly condition the signal. The circuit, on the other hand, requires a minimum level of capacitance for it to be able to effectively condition the output signal and this in turn affects the distance or gap required between the capacitor plates to produce the minimum capacitance level. In transducers of the type disclosed in U.S. Pat. No. 4,716,492 distances between the plates are in the order of 10-17 microns.
One approach described in the above patent to provide this selected gap employs a flat diaphragm element secured to the base substrate in selectively spaced relation thereto by disposing a spacing and securing medium such as a mixture of glass frit including a plurality of balls of glass of selected diameter between the flat diaphragm and the substrate at the periphery of the diaphragm. The glass frit is selected to be fusible at a first temperature at which the balls remain unfused and the mixture is then heated to the fusing temperature of the frit to secure the diaphragm to the substrate at a spacing from the substrate determined by the diameter of the balls. The provision of flat surfaces which extend over the entire diaphragm as well as the base substrate is very conducive to consistent, reproducible results from device to device; however, the flat surfaces generally require grinding to ensure that the surfaces are parallel to one another. Further, the use of the glass material to both space and secure the diaphragm to the base results in undesirable yield losses due to various factors such as unevenness sometimes occurring due to imperfections in the grinding process, variations in the compressive force used to clamp the diaphragms to the base when the device is fired to fuse the glass and other process variables such as the specific temperature profile of the firing and the specific glass composition employed.
In copending U.S. application Ser. No. 07/972,680, assigned to the assignee of the present invention, an improved, low cost pressure transducer is shown and described which comprises a body of ceramic material having a cavity formed therein closely adjacent an outer surface thereof. Metal capacitor plates are deposited on opposite sides of two surfaces defining the cavity with vias extending to terminal areas. The ceramic comprises conventional material such as 80% by weight alumina up to essentially 100% with the balance being additives to form a glass at a sintering temperature. The ceramic is provided in powdered form coated with an organic binder ready for pressing into any selected configuration. First and second portions, i.e., a diaphragm and a base having a recess formed in an outer face surface, are formed by pressing the powder in respective dies. Metallized coatings are deposited, as by screen printing, on one surface of the diaphragm portion and on the recessed outer face surface of the base portion. The vehicle used in the coatings is then removed, preferably by heating. Spacer means of organic material is optionally placed in the recess to ensure that the cavity gap is maintained during the following pressing step. The two portions are then pressed together to form a single unit and then the unit is heated in an air atmosphere to a first debinderizing temperature. After the organics, including the spacer means, are vaporized/decomposed and released through the still open cells of the ceramic, the unit is placed in a high temperature oven and co-fired in a reducing atmosphere with the metal layers forming a conductive coating bonded to the ceramic and the ceramic being sintered together to form a monolithic, closed cell body.
The metallized coatings on the base and diaphragm portions include a central plate portion in addition to traces or vias extending from the plate portion in a radial direction out toward the outer periphery of the body to a pad for connection to external terminal members.
These pads, however, are located within the monolithic body so that electrical connection to them must be effected by connecting an outside lead or terminal to the pad. This can be accomplished by forming a cut-out portion in a portion of the diaphragm portion extending to the pad. However, while the plate portion on the base can be connected to a terminal pad formed on the base at the cut-out portion it is difficult to provide a suitable electrical trace connecting the plate portion on the diaphragm with a corresponding terminal pad formed on the base. Further, it becomes cumbersome to attempt to make external electrical connections at this location and still provide a suitable package for the sensor which exposes the sensing surface to a fluid pressure source whose pressure is to be monitored.
Another way of providing electrical access is to form bores through the base portion aligned with the pads. After the body has been sintered, terminal pins are inserted in the bores along with suitable conductive epoxy to form a continuous electrically conductive path from the pin, through the conductive epoxy to respective pads. When the pad is disposed on the diaphragm portion the conductive epoxy is in direct contact with the major surface area of the pad thereby forming an efficient electrical connection; however, when the pad is disposed on the base portion it becomes more difficult to make a consistent, reliable electrical connection. Although it is possible to plate around sharp corners such as the corner formed by a bore and a surface around the bore, it would require chemical baths and a relatively large capital investment making the resulting device too costly. Screen printing is economical and suitable for applying the circuit traces and pads. However, attempting to print around a 90.degree. corner can result in reliability problems. That is, getting the pad to extend into the bore requires using a vacuum to pull the conductive material (ink) into the bore. The flow rate into the bore requires close control to ensure that the ink is not sucked off the corner by the passing gasses which would result in a discontinuous connection in the bore. A shallow ramp leading to the bore could be provided with the conductive layer printed on the ramp and the conductive epoxy received in the cut-out portion forming the ramp as shown in U.S. Pat. No. 4,972,717. However, use of a ramp involves more space than is desirable when making a small transducer. Further, when pressing the diaphragm and base together to a form a monolithic body it is difficult to obtain consistent geometrical spacing from device to device.